In the aeronautical field, for only mentioning a single field of application, carbon fiber composite parts are increasingly used for obtaining more and more lightweight structures without however abandoning rigidity and other required characteristics, but also for obtaining components which have a more pleasant aspect than a metal part while providing fire resistance.
The making of such composite parts requires high precision tooling. This tooling which may also be called a mold, should both provide the usual functions of any tooling, as recalled hereafter, guarantee geometrical tolerances of the final part and sometimes withstand rather particular thermal and pressure cycles such as those which are involved for example upon passing into an autoclave.
Usually, metal molds made in Invar steel by sheet metal work and machining or composite molds made in carbon fibers pre-impregnated with epoxy resin by molding in a mold called the “master”, are used for this purpose.
Although the Invar technology is widely used in mass production, it is nevertheless a fact that the unit cost of such a mold is high. In return, toolings made in carbon fibers are less expensive, but they are more fragile and practically unrepairable, and this mainly because of problems of leaks which subsist after their repair.
When the matter is of making cylindrical parts, notably with large dimensions, the problems for their making originate less from their size and from difficulties in handling such molds, but rather from the difficulty of guaranteeing the final geometrical tolerances of the part, because the latter depend on the behaviour of the mold during the polymerization cycle and also very strongly depend on the structure of the part to be obtained and on its behaviour on its tooling. Indeed, the problems resulting from thermal expansion and shrinkage of the resin are often not negligible.
In such a situation, only a machining approach would provide a guarantee on the final tolerances of the part.
There are composite materials which, molded and polymerized at a high temperature and under high pressure in an autoclave, may then be machined in order to obtain the desired geometry, while retaining the required seal for making the final part. However, such materials require the making of a “master” capable of withstanding the temperature and pressure conditions of their polymerization in an autoclave. Such “masters” are all the more costly and complex since the structure to be made is of large dimensions and that geometrical requirements are strict. As an example, tolerances of +/−0.6 mm for a diameter of the part of 4 m are frequently conditions to be met. Further, segmentation of tooling into several petals for allowing removal from the mold makes this use even more problematic.
As indicated at the beginning of this description, making a composite part in carbon fibers impregnated with epoxy resin requires high precision tooling, which should provide the usual functions of any precision tooling. In the field of interest here, the tooling should allow draping of a stiffened skin with profiles of the n type by depositing fibers. The tooling should also be able to be extracted from the draped structure after polymerization. Further, the tooling should comply with the required tolerances and should provide for holding stiffeners and their cores. Finally, the surface for depositing the fibers should be leakproof at room temperature and at high temperature in an autoclave in order to be able to place a vacuum bag during polymerization. The tooling should also allow attachment of cauls or counter-molds (caul plates) for the polymerization phase. And finally, the tooling should withstand the environment conditions of the method for making the part.
Moreover, a composite tooling intended for making a cylindrical part in a composite material should include thermocouples, should have a contact surface with a maximum roughness of Ra=0.8 and should integrate overlengths required for trimming the required part.